Additive metal deposition is an industrial technique that builds fully-dense structures by melting powdered or wire metal, via a laser or other energy source, into solidifying beads, which are deposited side by side and layer upon layer upon a workpiece substrate. It is known to utilize the process to repair and rebuild a worn or damaged component using a laser to build up structure on the component. The process is particularly useful to add features such as bosses or flanges on subcomponents of fabricated structures. The basic process involves adding layers to the component to create a surface feature on the component via the introduction of depositing material (delivered in the form of injected powder or a wire) into a laser beam. The additive process is known by several names including “laser cladding,” “laser metal deposition,” “direct metal deposition” or “additive metal layering.”
Additive metal layering is typically performed by using a computer aided design (“CAD”) to map the geometry of a part (known as a “build”) and then depositing metal, layer-by-layer, on the part. The CAD mapped geometry is input into a computer controlled (robotic) part handler that can manipulate the part in multiple axes of movement during the deposition process. In all of these techniques a heat source (typically an industrial laser beam) is used to create a melt pool into which a wire or powdered feedstock is fed in order to create beads upon solidification. In practice, the heat source is under computer numerical control and is focused onto a workpiece, producing the melt pool. A small amount of powder or wire metal is introduced into the melt pool, building up the part in a thin layer. The beam follows a previously determined toolpath. The toolpath is generated based on the CAD data that computes the needed build layer by layer. The beads are created by means of relative motion of the melt pool and the substrate, e.g. using an industrial robot arm or an XY-table. A part is then built by depositing the beads side by side and layer upon layer. The most popular approach combines a high-power laser heat source with metal powder as the additive material.
Careful tuning of the deposition tool and parameters, such as the powder or wire feed rate, the energy input, and the traverse speed are therefore important in order to obtain layers, which are free from defects such as shape irregularities, lack-of-fusion or cracks. Droplet forming, i.e. globular transfer of the molten metal, is also a common disturbance that affects the geometrical profile of the deposited beads and stability of the additive layers.
Creating an accurate geometric description of, and tool path for, the build to be fabricated is critical to system operation and achieving a high-quality layered end product. The currently known laser additive processes attempt to generate a geometric description using a homogeneous, full geometry representation of the part to program the in-process tool path. This current method is depicted in FIGS. 1A-1E. In FIGS. 1A-1E the build direction is represented by the arrow adjacent to the representative structure, which resembles a tuning fork in shape. In FIG. 1A a CAD model or a part 70 is developed. Then, in FIG. 1B the tool path software generates a slice section 71 of the CAD model representing a section that will be built according to the planned build direction. After the build section is generated, an algorithm creates a tool path 72 indicating the path of travel of the layering apparatus during the prospective deposition process. This is shown in FIG. 1C. Using the calculated tool path, the system can calculate laser power needed along the path (laser power schedule 73) as shown in FIG. 1D.
Using the current processes for generating a geometric description, the build process runs into a problem. This is shown in FIG. 1E. In this respect, during the build process the deposition apparatus encounters regions (points “X”) where the part is unsupported. To overcome this problem, existing additive process planners for three-axis processes require support structures for any unsupported geometry. These support structures must be made of a material than can survive the processing environment and is compatible with the build material. Typically this means that the support will be made of the same material as the part. The support structure solution adds cost due an increased volume of deposition and to increased machining necessary to remove the support. The desire to avoid machining large volumes of material is generally a major factor in choosing to produce a component by additive means. Hence, using a support structure is an undesirable characteristic of an additive process plan.